Electron Speed

[marshall4]

Find the final speed of an electron, starting from rest, passing between two parallel plates with a potential difference of 5000V.

m=9.1*10^-31kg
q=1.6*10^-19C

v=sqr root of [2(1.6*10^-19)(5000)]/9.1*10^-31 = 4.2*10^7 m/s

Is this answer valid, why???

[Adrian Baker]

Yes and No

The calculation is correct and in Classical Physics all is fine. However, if you increase the PD to say 30,000V you will find the electron travelling faster than the speed of light!

Your calculation doesn't account for the mass increase of the electron as it approaches the speed of light. For most School work it is fine though.